This invention relates generally to lasers and more particularly to lasers having passive pulse modulators.
As is known in the art, lasers, which have been used in a wide range of applications, include a gain medium disposed in a resonant cavity having a pair of end reflectors, or mirrors. After pumping the gain medium with energy from an external source, such as a flash lamp or another laser, spontaneous emissions are produced in the gain medium. Various modes are then coherently sustained in resonance within the cavity by stimulated emission.
In high peak power lasers applications, various modulation techniques have been used to produce a train of pulses having high peak power. Such modulation techniques include gain switching, Q-switching, mode-locking, and combinations thereof. In applications requiring high operating efficiencies, it is desirable that the pulse modulation technique utilize little or no power. One such modulator is a passive, saturable absorber, such as a multiple quantum well (MQW) saturable absorber, as described in articles by U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson and Mt. T. Asom entitled: "Passively modelocked Nd:YLF and Nd:YAG lasers using a new intracavity antiresonant semiconductor Fabry-Perot" published in the Advanced Solid State Lasers 1992 conference at Pd9-2 through Pd9-4; and, "Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable absorber", published in Optics Letters, Apr. 1, 1992, Vol. 17, No. 7 pages 505-507.
As is also known in the art, various techniques have been used to remove the light energy from the cavity. One technique uses a pair of mirrors at the ends of the cavity having very high, substantially total reflectivity, i.e., in the order of 99+% reflectivity. In such case, the output is taken from a point in the cavity between the pair of mirrors and a pair of outputs is produced. Such technique is described in the above referenced articles and with such described arrangement, the saturable absorber is fabricated on one of the pair of substantially totally reflecting mirrors. One technique used to form a reflective mirror is to form a distributed Bragg reflector. Such distributed Bragg reflector has a plurality of interleaved layers grown with different indices of reflection. However, in order to increase the amount of reflectivity, the number of layers grown must correspondingly increase.
In another technique, one of the pair of end mirrors is substantially totally reflective, as described above, to provide a rear reflector; however, the other end mirror has a reflectivity which is partially transmitting (e.g., 70 to 98% reflective) and provides the laser with a single ended output coupler. One such arrangement is described in an article entitled: "Passively Mode-Locked Er.sup.3+ Fiber Laser Using a Semiconductor Nonlinear Mirror" by W. H. Lob, D. Atkinson. P. R. Morkel, M. Hopkinson, A. Rivers, A. J. Seeds. and D. N. Payne, published in IEEE Photonics Technology Letters, Vol. 5, No. 1, Jan. 1993, pages 35-37. In such described arrangement, the gain medium is a doped fiber. The laser resonator is formed by butting one end of the doped fiber against a 50% reflective output coupler, while light exiting the other end of the doped fiber is coupled to the saturable absorber through an all-fiber wavelength division multiplexer (WDM) spliced to such other end of the fiber. In another arrangement, the saturable absorber is fabricated on a substrate and the substrate, with the saturable absorber formed on it, is then placed, or directly mounted, onto the output coupler. Such technique is described in an article entitled "Saturable Absorber Modelocked Polarization Maintaining Erbium-Doped Fibre Laser" by E. A. De Souza, C. E. Soccolich, W. Pleibel, R. H. Stolen. J. R. Simpson and D. J. DiGiovanni. published in Electronics Letters, 4th Mar. 1993, Vol. 29, No. 5 pages 447-449.
As is also known in the art, the fabrication of the laser must be relatively simple. Thus, while the arrangements described above may be adequate in some application, such arrangements are not adequate in applications requiring relatively low cost lasers adapted to operate with a relatively high efficiency (i.e. 1% wall-plug efficiency) to produce pulses of light having pulse durations of the order of 190 fs with a wavelength between 1.8 and 2.0 .mu.m from a pumping source having a threshold power of 18 mW.